Radiant tube infrared heaters are well-known in the heating industry and have numerous applications including heating buildings with high ceilings or frequent air volume changes. The use of heaters of this type is common because low energy costs are an important consideration for owners and users of industrial and commercial space. An important factor in achieving and determining heat and energy conservation is the need to measure and understand the heat being emitted from a heating source, such as a radiant tube heater. It is known that a low intensity infrared heater can have an average radiant efficiency from 35% to 40%, while a high intensity infrared heater can have an efficiency of 70%. Low intensity infrared heaters have a maximum operating temperature of about 1,200° F. while high intensity infrared heaters can operate at about 1,700° F. One advantage of an infrared radiant tube heater (RTH) is that it can readily burn natural gas which is the most environmentally acceptable fuel, particularly as it produces a low amount of pollutants when burned efficiently. A low intensity radiant tube heater normally consists of a fuel burner which can include a blower for combustion air, and a steel radiant tube which can range up to 70′ or more in length. A typical diameter for such a radiant tube is about 4″ and the tube can either be straight or bent (for example, to have a U-shape). An exemplary radiant tube heater has a sheet metal reflector covering the top and sides of the horizontal tube, this reflector partially shielding radiant and convection heat from escaping to a ceiling area above the heater. Thus, the reflector or shield reduces convectional losses and directs more radiant heat in the direction of the floor where it is needed.
Previous studies and papers have shown some of the advantages of a radiative or infrared heating system. They have shown, for example, that a radiative system uses less energy to achieve the same average mean radiant temperature compared to a convective system. It has been shown that a radiant heater can maintain comfortable conditions for occupants with less fuel consumption than warm air space heating units. A technical data sheet on infrared radiant heating by Naval Facilities Engineering Services Center, Port Hueneme, Calif., 93043-4370 (May 1966) discusses heat loss calculation in a building and the relation between RTH height and the coverage areas by providing rules of thumb for locating infrared heaters.
Feedback from heating customers has identified a need to measure heat flux and temperature at a specified distance from the centre of the radiant tube heater in order to assess the comfort level for persons in the vicinity of the RTH and provide better space heating designs. It will be understood that radiant tube heaters are positioned preferably to direct radiant incident rays towards the floor, humans and objects where maximum radiant heat flux is required. One needs to avoid directing incident rays towards a wall because this would result in heat energy being wasted through the wall.
In connection with the use of a radiant tube heater system, it is known that the first tube section of the radiant heating tube (for example, the first 10-foot section) reaches the highest temperature during operation of the heater. This fact must be considered when one determines the safe distance between the heater and persons or combustibles located in the area of the heater. This is one reason for developing a method and an apparatus capable of measuring the heat flux from a radiant tube heater and received over a selected measurement area.